Vh-ajm,
Good question, where to runup when none is provided.
They way I and most seasoned pilots would mostly tackle this is to start up and quietly idle away some where discreet and into wind and where your prop wash won't annoy anyone. And with a clean surface under the props.
A simple key to this is to consider this matter before you even get in the aircraft. As you might even want to wander over to a pre selected area and check the surface suitability for blue metal/gravel.
Now if you don't have the luxury of a clean surface under the props, set your idle nice and low say 700-800 RPM consistant with smooth running and lean to max increase in RPM. When the RPM comes up a bit which it should, reset to that 700-800 RPM again. Be aware even with proper leaning some engines will still lead foul at these low RPM's due still not being hot enough to activate the lead scavenging agents. If your in an aircraft that is a bit inclined to foul on the ground then choose 900-1000 RPM
Do all your checks bar the engine runup then taxi off, do the runup bit on the run and quickly ish, you need very little head in the cockpit for a mag check and prop cycle/feather check. Particularly with a bit of practise, if the taxi route is going to be along gravel do this brief runup while taxiing into wind if you can.
Grass is often better to run up over than asphalt as it has a great ability to impeed the votices that occour under the prop in addition to also impeeding the movement of gravel/stones that lie within it.
Your SOP's have the final word though.
Regards
M